While a remarkable advancement has been made with the genetic origin of diversity of antibody, the mechanism of generation of the specificity of antibody-antigen reactionsis far from clear. It is generally considered that sterochemical complementarity would determine the specificity. On the other hand, it is also reported that a substitution in amino acids in a distant region from the binding site of antibody profoundly affect the antigen-antibody interaction. Such mutual influence between distant regions is very characteristic of the hypothetical global cooperative interactions which are also sensitive to substitution in amino acids. Being consistent with this view, studies using 1H-2H exchange, show that a polyclonal antibody exhibits a reduced motility and an increased stability without a large change in conformation upon binding with a hapten. Thus, we propose to investigate the specificity of antigen-antibody interaction in relation to the global cooperative interactions as follows. 1) A monoclonal antibody will be prepared to a fragment complex of cytochrome c. Then, the antigenic site will be determined. 2) Thermodynamic and kinetic parameters of this antibody-antigen reaction will be determined as a function of substitution in amino acids of the antigen using chemical synthesis of the fragments. 3) Change in molecular motion of antibody upon binding with antigen containing unsubstituted and substituted amino acids will be determined suing 1H-3H or H-2H exchange. The degree of these changes will be related to the strength of binding with antigen. 4) NMR studies will also be carried out in order to investigate modulation of concerted molecular motion of antibody by binding with antigen. 5) A fragment exchange technique will also be used to investigate influence of antigen-antibody interaction on interactions within the fragment complex (antigen). 6) Gene manipulation and cloning will be used to investigate the effect of substitution in amino acids of variable domain of light chains (and heavy chain, if possible) on the antigen-antibody interaction.